1. Field of the Invention
The present invention relates to a display device and a manufacturing method thereof. In particular, the present invention relates to a display device in which a single crystal semiconductor film is used and a manufacturing method the display device.
2. Description of the Related Art
In recent years, integrated circuits using an SOI (silicon on insulator) substrate in which a thin single crystal semiconductor film is formed on its insulating surface, instead of a bulk silicon wafer, have been developed. Since parasitic capacitance between a drain of a transistor and a substrate is reduced by using the SOI substrate, the SOI substrate has attracted attention as one improving performance of semiconductor integrated circuits.
One of known methods for manufacturing an SOI substrates is a Smart Cut (registered trademark) method. An outline of the method for manufacturing an SOI substrate by a Smart Cut method is described below. First, hydrogen ions are implanted into a silicon wafer by an ion implantation method and an ion implantation layer is formed at a predetermined depth from a surface. Next, the silicon wafer into which the hydrogen ions are implanted is bonded to another silicon wafer with a silicon oxide film interposed therebetween. Then, heat treatment is performed and the ion implantation layer becomes a cleavage plane and the silicon wafer into which hydrogen ions are implanted is separated into a thin film, so that a single crystal silicon film can be formed over the silicon wafer which is to be a base substrate.
A method in which a single crystal silicon film is formed over a supporting substrate formed from glass by such a Smart Cut method has been proposed (for example, see Reference 1: Japanese Published Patent Application No. H11-163363). Since glass substrates can have a larger area and are less expensive than silicon wafers, the glass substrates are mainly used for manufacturing liquid crystal display devices and the like.
However, in general, the size of a silicon ingot or a silicon wafer which is to be a base material is small compared to the size of a glass substrate which can be formed. Therefore, when a display device is manufactured using a single crystal semiconductor film formed over a glass substrate using a Smart Cut (registered trademark) method, it is difficult to enlarge a display portion.
Accordingly, when a display device provided with a large display portion is manufactured using a single crystal semiconductor film, a plurality of silicon wafers is required to be bonded to a glass substrate having a large area. For example, a technique for making small pieces of single crystal silicon into a tiled pattern over a glass substrate is disclosed as for an SOI substrate for an active matrix liquid crystal display in Reference 2: Japanese Published Patent Application No. 2005-539259.
In order to improve a resolution of a display region in a display device while keeping the same display area, the size of a pixel is required to be reduced. When the size of the pixel is reduced, an interval between thin film transistors (TFTs) each provided in a pixel is reduced logically. However, since silicon does not exist in a gap (a joint portion) of a plurality of single crystal silicon provided over a glass substrate by bonding, a thin film transistor cannot be manufactured at the joint portion. Therefore, improvement in the resolution (higher definition) of the display portion is difficult.
Further, an edge portion of a semiconductor substrate is rounded generally, and a bonding at the edge portion cannot be performed sufficiently (for example, see Reference 3: Japanese Published Patent Application No. 2001-345435). Accordingly, even when a plurality of semiconductor substrates is arranged so as to be adjacent to each other, it is very hard to eliminate the joint portion between the semiconductor substrates completely.